The invention relates to the reproduction of three-dimensional images and, particularly, moving images.
Known systems for recording and reproducing images with depth can be divided into two major groups: holographic systems and non-holographic systems.
Amongst the latter are stereoscopic and so-called three-dimensional systems, which differ from one another. The term stereoscopic (also sometimes called stereographic) is used for systems in which only two images, taken at a distance apart approximately equal to the average distance between human eyes, are used in the reproduction. The term three-dimensional (also sometimes called panoramagraphic) is used for systems in which a greater number of images are used in the reproduction. (The field of this invention is that of three-dimensional reproduction, as has just been defined, in motion.)
More specifically, holographic systems are based on the reconstruction of wavefronts. They require, therefore, coherent light sources, at least for image taking. Temporal coherence requires the light to be monochromatic. Spatial coherence requires the light to come from a point source. Therefore the development of these systems has been intimately linked with the development of the laser, since its light is intense and highly coherent.
Laser techniques for holography are complex and expensive. Great technical difficulties have to be overcome. This has hindered the commercialization of holograph systems. Holographically photographing distant objects, like the moon, is difficult, because such objects are difficult to light up with a coherent beam. It turns out, too, to be impossible to photograph sunsets or reflections of the sun or moon on the sea, landscapes, etc. Finally, because observation through a transparency is necessary, the size of the images that can be reproduced is limited.
Stereoscopic systems are based on bringing a different image to each eye of an observer. For this, two photographs are taken with two cameras, the objectives of which are separated from one another by a distance approximately equal to the average distance between human eyes.
There are various systems for reproducing the photographed images, depending on the procedure used to bring the image taken by the lefthand camera to an observer's left eye and that taken by the righthand camera to the right eye. They all require the observer to be provided with an optical, electronic or electro-mechanical appliance in front of his eyes, however, such as coloured, polarized or shuttering filters.
The systems with coloured filters (anaglyphs) bring a different image to each eye by placing a filter in front of each of the observer's eyes, red (or yellow) for one eye, green (or blue) for the other, each corresponding image being reproduced in red (or yellow) or in green (or blue).
In the systems with polarized light, polarized filters are respectively placed before the observer's eyes. The planes of polarization of the filters are perpendicular to one another. The planes of polarization of the light which reproduces the images are the same as those of the observer's filters.
In the shuttering systems, shutters are placed before the observer to interrupt the vision of each of the observers eyes. These leave each eye a viewing time that is complementary to that of the other. The images are also so reproduced alternately.
The principal limitation of stereoscopic systems in projection is that they require the observer to be inconvenienced by placing before him the coloured filters, polarized filters or shutters described above.
Other stereoscopic systems also exist in which bringing a different image to each eye is achieved, however, by procedures which are not suitable for projection. Amongst these are those which place an optical system between the observers and the reproduced image, such as the Brewster prism method, the Wheatstone flat mirror method or Kemp's concave mirror method, U.S. Pat. No. 4,623,223.
Some known three-dimensional systems of reproduction using ordinary light are capable of reproducing three-dimensional images in motion. All of the three-dimensional moving-image reproduction systems developed up till now, however, use a diffusion surface on which the various images are generated, projected, transmitted, amplified or, simply, printed. Typical in such image generation would be a cathode ray tube screen projection onto an opaque or translucent diffusion surface.
It is important to emphasize one characteristic that is common to any diffusion surface, because it greatly affects the design of all the three-dimensional reproduction systems that use this type of surface: every point on the diffusion surface is a point source of light radiating photons in all transmitting directions. As a result, any observer, whatever his position, will see the whole image reproduced from every point on the diffusion surface.
If two or more images for a three-dimensional reproduction system were reproduced at the same time at the same point of the diffusion surface, therefore, the photons coming from the different images would appear mixed together to any observer in any direction, that is, at least, there would be no three dimensionality. For this reason, the different images reproduced on the diffusion surface of every three-dimensional reproduction system are distinguished by reserving a different place on the diffusion surface (screen) for each, that is, by scalar image differentiation. The different places on the screen usually are very fine vertical stripes.
U.S. Pat. No. 4,737,840, the three-dimensional reproduction procedure used is projection through a vertically striped shield plate onto the diffusion surface.
In Haisma's U.S. Pat. No. 4,571,616, the three-dimensionally reproduced images are also vertically striped. In this case, however, the images are positioned on the diffusion screen by being guided along light conductors.
In all these cases, viewing is through an optical sheet of cylindrical lenses, that is, lenticular sheet. The focal lines of the cylindrical lenses are on a plane at which the diffusion surface is situated. The focal length of the cylindrical lenses can be short, such as about a millimeter, for example, so that the cylindrical lenses and diffusion sheet can be, at least practically, one.
It is important to bear in mind, too, that, in all such vertically striped three-dimensional reproduction systems, the transverse dimension of each vertical image stripe must be "n" times smaller than the transverse dimension of each cylindrical lens, wherein "n" is the number of vertical-stripe images to be reproduced. For this reason, the size of the cylindrical lenses is limited by that of the images, which are, in turn, "n" times smaller than the lenses.
The quality of the three-dimensional image reproduced is, however, lowered as the transverse dimension of the cylindrical lenses increases. The latter is, however, limited by the minimum practical transverse dimension of the vertical stripes for the image for each cylindrical lens.
The maximum viewing angle is also limited by the aperture of each cylindrical lens, which depends upon the relationship between the transverse dimension of the cylindrical lens and its focal length. If the viewing angle is exceeded by an observer, his observation includes an image stripe for an adjacent cylindrical lens, which produces an undesirable, so-called pseudoscopic effect, that is, inverted depth.
To ensure a good-quality reproduced image from such three-dimensional systems, it is also necessary:
1. That there be no space between each two adjacent cylindrical lenses; the cylindrical lenses must contact one another;
2. That the transverse dimension of the cylindrical lenses be small enough to be imperceptible; and
3. That the variation of the horizontal parallax appear to be continuous over a viewing angle wide enough so that pseudoscopy does not occur for any observer.
The 1st condition requires consideration of the maximum viewing angle without pseudoscopy. It is expressed by: ##EQU1## which for ordinary materials, which have refractive indices of around 1.5, takes a value of approximately 54.degree. or .+-.27.degree. C. to a perpendicular from the diffusion surface. The preservation of this angle across a diffusion screen of reasonable size requires many precise correspondences between each of the many contactingly adjacent cylindrical lenses that are required and its image (group of "n" stripes), which is difficult to achieve and, therefore, expensive to manufacture.
The 2nd condition, which requires the transverse dimension of the cylindrical lenses to be small enough to be imperceptible requires transverse dimensions "d" for a healthy eye of: ##EQU2## This indicates, for example, d=0.3 mm. for a viewing distance of 1 m. and d=0.08 mm. for a viewing distance of 0.25 m. If 10 image stripes are used for each cylindrical lens, the transverse dimension of each image stripe then has to be 0.03 and 0.008 mm., respectively. These values are on the order of only 15 times the wavelength of visible light. The difficulties of making an image-stripe system for this, particularly keeping in mind the 1st condition, too, are obvious and, therefore, the price of a commercial product would be high.
The 3rd condition suggests a viewing angle greater than the 54.degree. that can be achieved with ordinary materials, as discussed above with respect to the 1st condition. It also suggests the use of such fine image stripes that the manufacture of devices for producing them would not be viable.
These three conditions explain why such three-dimensional dimensional reproduction systems have not been brought onto the market successfully, not even for cinemato-graphy with small projection screens.
There are also so-called integral three-dimensional reproduction systems, which are capable of reproducing both horizontal and vertical parallax simultaneously. We owe their invention to Lippmann, the famous French optics scientist, in 1908. Their basis is a fly's eye lens sheet of glass or plastic, generally with a tremendous number of spherical plano-convex lenses (for example 10,000).
Examples of such integral three-dimensional projection reproduction systems are disclosed in Ando's U.S. Pat. No. 3,852,524. The Ando patent does not mention, however, the number of images required nor the band width required for their transmission. It says simply that they are multiple and transmitted by a carrier of very high frequency. In fact, however, the processes of image recording and reproduction require the handling of an enormous amount of information, because every plano-convex lens requires a full two-dimensional image and thousands of plano-convex lenses must be used. In addition to these problems, and that of using spherical optics (the fly's eye lens sheet), image reproduction is always carried out in all the systems described in the patent through a diffusion screen, which presents all the drawbacks of this already described.
The Haisma U.S. Pat. No. 4,571,616 already described also discloses an integral three-dimensional system based on images from conventional cameras in square mosaics of 3.times.3, for example. Reproduction is done by positioning the nine square mosaic images behind each spherical lens of a fly's eye lens sheet. The patent says that this is achieved by appropriately positioning optical conductors with mechanical means, but the complexity of manufacture previously considered for positioning "n" vertical image stripes behind each of several cylindrical lens then becomes much more serious, because it then involves positioning "n.sup.2 " squares of images behind each spherical lens. In addition, the system described by the Haisma patent still requires a diffusion screen, in this case composed of the ends of optical conductors.
Some other fields of application exist, in robotics, for example, for optical systems of spherical lenses (fly's eye lens sheets) as described, for example, in Stauffer's U.S. Pat. No. 4,410,804. These propose, however, only obtaining data on the range of objects and their shapes, and not three-dimensional image reproduction systems providing both vertical and horizontal parallaxes.